We are trying to develop new molecular paradigms for the regulation of gene expression in bacteria. To this end, we are studying the regulation of the histidine operon of Salmonella typhimurium and the regulation of the isoleucine and valine operon in Escherichia coli K12. The histidine regulation project is divided into two sub-projects: the origin and replication of the pi-histidine plasmid and the role of PRPP-ATP ribosyl transferase in histidine regulation. The isoleucine and valine project consists of examining the role of threonine deaminase in the repulatory process, the role of isoenzymes in a biosynthetic pathway and the identification of other regulatory elements in the process. BIBLIOGRAPHIC REFERENCES: Meyers, Marilyn, Blasi, F., Bruni, C., Deeley, R.G., Kovach, J. Levinthal, M., Mullinix, K. Vogel, T. and Goldberger, R.F. Specific binding of the first enzyme for histidine biosynthesis to the DNA of the histidine operon. Nucleic Acid Res. 2: 2021-2036, 1975. Levinthal, Mark, Levinthal, Maxine, and Williams, Luther. The Regulation of the ilvADGE operon I. Evidence for positive control by threonine deaminase. J. Molec. Biol. in press 1976.